The present invention relates to a method for forming bags from plastic material.
It is known that various products such as granular products and the like are packed in plastic material bags which are hermetically sealed by means of heat sealing. These bags are employed for both product transportation and storage and generally bear on the outside the information for proper identification of their contents.
Presently, these bags are made starting from a tubular element of plastic material in a sheet, which is folded so as to form gussets along its opposite sides and flat wrapped in a roll. Individual portions, each bearing a crosswise seam which defines the bottom of the bag to be formed, are then cut from the roll.
FIGS. 1 to 3 illustrate a prior art bag of this type. As shown in FIG. 1 for greater clarity, the bottom seam 2 extends to the overlapping sheets 3 and 4 of the tubular element 1 across the entire width of the same. Along the side edges, however, the seam also extends to flaps 3a and 4a folded inwards so as to form a gusset within sheets 3 and 4. In such circumstances the sealing conditions between the central area of the tubular element 1 where only the two sheets 3 and 4 are to be sealed together, and the side sections where four overlapping sheets are to be joined, that is sheets 3 and 4 as well as their relevant flaps 3a and 4a folded inwards to form the gussets, are markedly different. Seams made under these sealing conditions are far from ideal for both the central and the side sections of the gusseted tubular element.
This is rather a makeshift solution as the seam joining the central and side sections is made under differential sealing conditions, so that resistance at the critical point joining these two sections and indicated by C in FIG. 1 is weaker.
So as to at least partially overcome this drawback, that is to improve resistance at the aforementioned critical point C, additional angular seams 5 are made, which are symmetrically arranged at the bottom and at the top of the bag.
These angular seams 5 are made in the section of the tubular element where the portion of the material defining the bag is subsequently cut and involve simultaneously the top of the preceding bag and the bottom of the subsequent one.
The angular seams 5 at the top of the bag cause a constriction of the opening 6 through which the bag is filled, as can be seen in FIG. 2 where 7 indicates the gripping elements which widen the opening 6. This constriction causes a proportional reduction in the amount of product which may be introduced through the opening 6 at a time, i.e., in the filling rate, and therefore in overall productivity. To eliminate this drawback, an attempt has been made to improve the filling rate by increasing the height from which the product was dropped into the bag, albeit this increased the risk of product spillage and so on.
The gusseted tubular element is then sealed by means of a second crosswise seam 8 which defines the top of the bag as shown in FIG. 3 where 9 indicates the finished bag thus obtained. The same considerations made as to the reduced resistance at critical point C owing to the sealing of the gusseted folded flaps also applies to seam 8 at the top of the bag.
It should also be pointed out that the bags are easily subject to blows, falls and other similar events during handling and that such circumstances are extremely testing on the resistance of the aforementioned critical point.
Another drawback inherent in traditional gusseted bags is associated with the need to print identification data on to their outer surfaces. In order to do this, the surface which has to bear the printed data must be pre-treated accordingly so as to ensure that the ink to be applied will not come off. This treatment however hampers the perfect sealing of the plastic material so that it should preferably be avoided, or at least limited, in the folded side areas which are precisely those of the critical point where seam resistance is weaker.
In other words, there is the risk that any information printed in these areas will not be permanent so that there is no choice but to print the information on the front or back of the bags.
Such a limitation however detracts from the visibility of the printed information especially when the bags are stacked in piles or stored on shelves. In such circumstances, in fact, the information should ideally be printed on the sides so as to be easily and clearly visible and legible.
The object of the present invention is an easy and economic method for producing bags from plastic material which are highly resistant in particular in correspondence to the closing seams at the top and bottom.